The noise generated by gas turbine engines can be broadly classified into three categories, namely, source noise such as fan and compressor noise, core noise, and jet noise. The first of these, fan noise, is expected to be the most important community noise source for next-generation subsonic aircraft engine technology. The reduction of aircraft community noise, i.e., the noise impact on residential communities, has become a major consideration in subsonic aircraft engine design. Since the 1960's, when the aircraft industry moved to favor high bypass ratio turbofan engines in an effort to increase engine efficiency, there have been only evolutionary advances in methods to reduce aircraft noise. But as regulatory standards for community noise continue to tighten, quieter higher thrust engines are required. In particular, a goal of reduction in current-day engine noise levels by an amount corresponding to a 6 dB effective perceived noise level (EPNL) has been established by the NASA Advanced Subsonic Technology Program. Conventional noise reduction technology is insufficient in meeting this goal.
For the case of subsonic fan tip speed, gas turbine engine fan noise generally is dominated by unsteady loading on both the stator/pylons and rotor. This unsteady loading arises from interaction of the stator/pylons and rotor with both random and periodic gusts introduced by a variety of sources. The most significant sources for these gusts are the blade wakes and tip clearance flows. Specifically, the interaction of the rotor blade wake and rotor blade clearance flow with the stator are expected to be a dominant noise sources on the next generation of gas turbine engines.
The fan rotor wake of a high speed gas turbine engine differs considerably from that associated with a two-dimensional, low speed airfoil. In the latter case, rotor wake-stator interaction, as well as unsteady stator loading due to interaction with endwall flows, are classically considered as periodic phenomena. In contrast, in high speed fans such as gas turbine engines, these phenomena are sources for pressure fluctuations with rich spectral content. Indeed, the wake structure of high speed compressor and fan blades has been the subject of considerable study due to their influence on stage efficiency as well as their impact on the interpretation of compressor performance measurements.
The blade wake structure produced by high speed fans is primarily determined by two design characteristics of modem machines, namely, the large variation in spanwise boundary layer loading resulting from a low hub-to-tip ratio geometry aimed at uniform spanwise total pressure rise, and the high degree of unsteadiness resulting largely from the high boundary layer loading and shock wave boundary layer interaction on sections of the blade designed to have supersonic speeds. These design characteristics thus produce a highly unsteady blade wake. In the case of a gas turbine engine compressor rotor, the interaction of the unsteady rotor blade wake velocity field with a down-stream stator results in an acoustic field that is harmonically much richer than would be expected from the classic airfoil wake-stator analysis.
The source of high-speed blade wake unsteadiness is generally agreed to be associated with two boundary layer phenomena, namely, vortex shedding in the wake, and diffuser instabilities. Vortex shedding in the wake is a condition similar to that of a von Karman vortex street except that it is quite three-dimensional, and that the shedding frequency is time-dependent. The vortex shedding is modulated at low frequencies, i.e., at about 1/10 rotor blade passing, by diffuser-like instabilities within the blade passages. Compressor rotor blading is typically designed using a two-dimensional diffuser criterion to be just at the point where the diffuser pressure recovery drops precipitously, i.e., in the regime of diffuser instability.
Several methods for reducing fan noise have been proposed; e.g., Groeneweg et al., in "Aeroacoustics of Flight Vehicles: Theory and Practice; Vol. 1: Noise Sources; Turbomachinery Noise," NASA RP 1258, August, 1991; suggests the addition of fan duct acoustic treatments to absorb noise, the design of rotor blades with minimum blade section drag at operating conditions where noise levels are critical, the design of fans with a rotor-to-stator spacing that is large enough for the rotor wakes to decay and mix before impinging the stator blades, increase in the number of rotor wakes, and increase in the stator chord to reduce the unsteady lift response associated with wake-stator interaction. These methods are generally employed to some extent in current gas turbine engine technology, but are insufficient to significantly reduce radiated engine noise, and are not capable of meeting the 6 EPNdB noise reduction goal set by NASA.